Leakage of electric charge must be avoided in a variety of kinds of electronic devices that have been developed and put into practical use in recent years, such as organic electroluminescent (organic EL) devices, solar cells, touch panels, electronic papers, etc. Therefore, a high degree of water barrier property is required for the plastic base materials that are used for forming circuit boards and for the plastic base materials such as films that are used for sealing the circuit boards.
The water barrier property can be realized by providing a hygroscopic coating in which a hygroscopic agent is dispersed.
For instance, a patent document 1 is disclosing a gas barrier laminate obtained by forming an inorganic barrier layer on the surface of a plastic base material, and forming, on the inorganic barrier layer, a sealing layer (hygroscopic film) in which nano particles such as of a metal oxide or carbon nano tubes are dispersed as a hygroscopic agent.
Further, a patent document 2 proposes a gas barrier laminate (film) obtained by forming an inorganic barrier layer, an organic layer and a water-trapping layer (hygroscopic film) on abase film. Here, a hygroscopic material such as silica gel or aluminum oxide is dispersed in water-trapping layer (hygroscopic film) and in a high molecular binder such as polyamide.
Further, a patent document 3 discloses a gas barrier laminate comprising a plastic base material on which are deposited a gas barrier film and a hygroscopic layer (hygroscopic film), the hygroscopic layer containing an alkylene oxide, nano particles of acrylate or an organic metal complex.
While studying many such hygroscopic films, the present inventors have discovered that if a matrix of an ionic polymer contains dispersed therein a hygroscopic agent having such a hygroscopic property that the humidity thereof that is attained is lower than the humidity of the matrix, then a very high water barrier property is exhibited, and have previously filed a patent application (PCT/JP2014/052788). Then the inventors have furthered the study and have discovered that if a crosslinked product of a monovalent metal salt of an acrylic acid is used and if the hygroscopic agent is added in a predetermined range, then the hygroscopic film exhibits a high hygroscopic property (water shut-off property) as well as excellent transparency and surface smoothness.